Officers are appealing for witnesses to the collision, which happened at Junction 7 of the M25 (anti-clockwise) yesterday evening.
A spokesman said last night: “Shortly after 6.15pm this evening (October 27), we received multiple reports of a collision on the slip road between Junction 7 of the M25 (anti-clockwise), and Junction 8 of the M23.
“A silver Peugeot 306 had collided with a barrier and a lamppost, before coming to rest on the slip road to the M23.
“The driver of the Peugeot, an elderly man, has been taken to hospital with life threatening injuries. The passenger in the vehicle is also at hospital with minor injuries.
“The road was closed for a substantial period of time to allow the emergency services to do their work, and we are grateful to the public for their patience and understanding while this work is carried out.
“Two lanes of the M25 clockwise were reopened shortly after 7pm, with a diversion via junction 8 (doubling back onto the clockwise carriageway) and the clockwise sliproad for vehicles wanting to use the M25 to reach the M23.
“The anti-clockwise slip road to the M23 is likely to remain closed for a considerable period while repairs are made to the barriers and lamppost.
“Officers are keen to speak to anyone who saw the collision or the manner of driving of the Peugeot prior to the collision.
“They are particularly keen to speak to anyone who may have captured the incident on dash-cam or helmet-cam.
“Anyone with information that could assist the investigation into this collision is asked to call Surrey Police on 101 urgently, quoting reference PR/18258499.”
Motorists travelling to Gatwick have been advised to allow extra time.